This invention relates to the field of isolators for RF circuits and, more particularly, to an isolator having circuitry and "package" integrally formed.
Two types of isolation devices have been developed and used for providing one-way signal paths, namely, terminated circulators and resonance isolators. Circulators, typically, have three or more ports, with a minimum of signal attenuation between signals entering at a first port and leaving at a second, or entering at the second port and leaving at a third, but great attenuation in signal between the second port and the first. Thus, with the proper impedances at each port, a non-reciprocal device is provided. Also known are resonance isolators which are two-port devices utilizing the gyromagnetic resonance of a ferrite material, but these are efficient isolators only for a very narrow band of frequencies at resonance of the gyromagnetic material. Since the gyromagnetic resonance of ferrites is very temperature sensitive, this type of isolator requires careful control of power loss dissipated in the ferrite to prevent change of resonant frequency, or even increasing the ferrite temperature beyond the Curie point where the material becomes simply paramagnetic.
In a U.S. patent application, Ser. No. 682,686 now U.S. Pat. No. 4,016,510, assigned to the same assignee as is the present invention, a broadband isolator is disclosed. In this patent application, two conductors or loops with one end grounded are placed within a static magnetic field with their main axes perpendicular to each other. Also within the field and placed adjacent to the loops or lines are one or two ferrite discs, the field being normal to the planes of the discs and to the axes of the conductors. An electromagnetic shield box wraps around the conductors and discs and a high permeability return path is provided. A unilateralizing resistive element is coupled between the input and output terminals. This resistive element, being essentially nonreactive, provides the broadband response characteristic.
A practical model of such a broadband isolator for much higher frequencies, however, must take into account additional factors. For example, the free space inductance of the loops or lines is not negligible, therefore the resistive elements cannot be located at the ideal points in the network. Also, the capacity between the loops or lines becomes appreciable and must be allowed for. The effect on the network of the "package" or shield box can no longer be ignored, e.g., ground paths may become inductances and "good" grounds no longer are satisfactory. The ideal structure then appears to be a solid conductive block, carved out and formed to provide the necessary circuit elements, and requiring no external elements.